Use of polysilsesquioxane without hydroxyl group for forming mask

ABSTRACT

A high-energy radiation-sensitive pattern-forming resist material consisting of polysilsesquioxane having no hydroxyl group in its molecule. The pattern-forming material of this invention has an improved sensitivity to high-energy radiation exposure, a high resistance to dry etching, a high resolution capability, and an improved thermal stability.

This is a divisional of co-pending application Ser. No. 739,158 filed onMay 30, 1985.

BACKGROUND OF THE INVENTION

The present invention relates to lithography, more particularly, to apattern-forming resist or resist material suitable for forming a highquality resist pattern on a substrate or base material, usually asemiconductor, in the production of semiconductor integrated circuitsand other semiconductor devices, for example, large-scale integratedcircuits and bubble memory devices. The pattern-forming material issensitive to high-energy radiation such as electron beams, X-rays, softX-rays, and ion beams and is thermally stable.

A plurality of pattern-forming resist materials are available for thepurpose of recording the high-energy radiation described above. Inaddition to single resist layers, further, there are multi-layeredresist coatings such as duplitized or two-layered resist coatings orthree-layered resist coatings. A multi-layered resist coating is usefulin the formation of fine patterns on an uneven substrate using, forexample, submicron electron beam lithography. This is because, asdescribed hereinafter, the multi-layered resist coating effectivelydecreases scattering of the electron beam and its adverse influence onpatterning, and/or a proximity effect. The differences in effect betweena single resist layer and a duplitized resist coating in the electronbeam lithographic process will be described with reference to FIGS. 1and 2.

FIG. 1 illustrates steps of a pattern formation process using a singleresist layer. A substrate 1 has a single layer 2 of the resist materialapplied thereon. As shown in FIG. 1(b), the resist layer 2 is irradiatedwith a pattern of electron beams (e⁻). During the patterning process,the electron beams are undesirably scattered within the resist layer 2.(Scattering of electron beams is shown by arrows a and b.) Scattering ais caused due to the properties of the resist layer 2, while scatteringb is caused by back scattering of the electron beams from the substrate1.

The illustrated scattering of the electron beams adversely affects theaccuracy of the resulting resist pattern. For example, if the resistlayer 2 has a relatively high thickness, the resulting resist patternwill show extension of the pattern ends. Further, if the layer 2 isrelatively thin, the width of the resist pattern will be increased dueto back scattering of an electron beam once striking the substrate 1. Inboth of these cases, it is impossible or difficult to form fine resistpatterns on the underlying substrate. The insufficient and undersirablepatterning of the resist layer is apparent from FIG. 1(c), across-sectional view of the developed resist layer 12.

FIGS. 2(a) to (c) show a typical example of the use of the duplitizedresist coating. The principle of the illustrated method is alsoapplicable to a pattern-forming process of this invention. As shown inFIG. 2(a), the substrate 1 has applied thereon a lower resist layer 3and an upper resist layer 4 thinner than the layer 3. The thicker layer3 is sandwiched between the substrate 1 and the thinner layer 4 and isformed from an organic resin having no sensitivity to the energyradiation used during the patterning of the layer 4. The layer 3 isfurther effective to level an uneven surface of the underlyingsubstrate, for example, the surface of LSI chips, and therefore isgenerally referred to as a leveling layer.

Upon electron beam exposure, the exposed area of the upper resist layer4 is insolubilized due to cross-linking of the resist material (seereference number 14 of FIG. 2(b)). Development is then carried out toremove the unexposed area of the resist layer 4. The patterned resistlayer 14 is obtained. The pattern of the layer 14 is transferred to theunderlying layer 3 by dry etching the layer 3 through the patternedlayer 14, which acts as a mask or masking element. The patterned resistlayer is shown in FIG. 2(c).

As is apparent from the above description and the accompanying drawings,the duplitized resist coating (3 plus 4) is free from scattering of theelectron beams, since the upper layer 4 is very thin and the lower layer3 is not affected by the electron beams during patterning of the upperlayer 4. This effectively diminishes lateral extension of the patternwidth. The effect is greater along with lesser layer thicknesses of thelayer 4. Further, increase of the layer thickness of the lower layer 3diminishes the influence of back scattering of electron beams onto thepatterning, thereby resulting in a decreased proximity effect. As aresult, fine resist patterns with a high accuracy and a high aspectratio can be obtained. The term "aspect ratio" used herein, as isgenerally recognized in the art, means the ratio of the layer thicknessto the pattern width of the resist pattern. A high aspect ratio meansthat the resist pattern has a high accuracy of size.

However, no satisfactory resist material for the formation of the upperlayer of the duplitized resist coating has yet been proposed. A priorart upper-layer forming resist material is chloromethylatedpolydiphenylsiloxane of the structural formula: ##STR1## which isreferred to as SNR in this field. The resist material has a highresistance to oxygen plasma etching used in the etching of theunderlying resist layer and shows an electron beam sensitivity of about5 μC/cm² and a submicron resolution capability. Reference should be madeto EP No. 122,398-A and M. Morita et al: "Silicone-type negative-workingresist SNR (I)", 44th Symposium Preprint, 28a-T-1, Japan Society ofApplied Physics, P. 243, Sept. 1983. Another prior art upperlayer-forming resist material is P(SiSt-CMS) reported in N. Suzuki etal: "Resist material for duplitized structure", 44th Symposium Preprint,26a-U-7, Japan Society of Applied Physics, P. 258, Sept. 1983.P(SiSt-CMS), namely, the copolymer of trimethylsilylstyrene andchloromethyl styrene of the structural formula: ##STR2## is resistant tooxygen plasma and has an electron beam sensitivity of about 4 μC/cm² anda submicron resolution capability. Both of these resist materials,However, cause corrosion of the underlying aluminum or other metalcircuit because they contain chlorine atoms.

Another resist material suitable for the formation of the upper layer ofthe duplitized resist coating is reported in M. Hatzakis, J. Paraszczakand J. Shaw, in "Proceedings of the International Conference onMicrolithography (Microcircuit Engineering '81, Lausanne)", P. 386(1981). PDMS reported therein, namely, dimethylsiloxane of thestructural formula: ##STR3## is resistant to oxygen plasma and has anelectron beam sensitivity of about 2 μC/cm² and a resolution capabilityof about 0.5 μm l/s. There is no problem about the corrosion of metalwiring or the circuit because PDMS does not contain a chlorine atom inits molecule. However, PDMS is generally oily or gummy at an ordinarytemperature, it is difficult to obtain a uniform and thin coating of theresist.

The research staff of Fujitsu Limited found that silicone resins havinga ladder structure, particularly polysilsesquioxane, is highly sensitiveto high-energy radiation such as electron beams or X-rays and is highlyresistant to reactive ion etching, plasma etching, sputter etching, orother dry etching and therefore is useful as a pattern-forming resistmaterial (Japanese Unexamined Patent Publication (Kokai) No. 56-49540).The resist material can be effectively used in the formation of theupper resist layer discussed above, but there are several difficultieswhen it is used in such a lithography process. First, the describedresist material easily hardens when its coating is heated to evaporatethe solvent therefrom. The hardened resist coating is insoluble in thedeveloping solution and therefore cannot be used in the subsequentpatterning steps. Heating of the coated resist material at a relativelylow temperature is not desirable since it means a longer processingtime. Second, the described resist material is thermally instable and,therefore, cannot be stored for a long period without change of itsproperties. Third, it is impossible to prepare a monodispersed sample ofthe resist material which shows a high resolution capability. This isbecause the dependence of the solubility of the resist material orpolysilsesquioxane on its hydroxyl equivalent and molecular weight makesthe fractional precipitation process necessary for such preparationdifficult.

Therefore, what is now desired is a high-energy radiation-sensitivepattern-forming resist material having improved sensitivity to electronbeams, X-rays, proton beams and other high-energy radiation exposure,resistance to reactive ion etching, sputter etching, plasma etching, andother dry etching, improved resolution capability based onmonodispersibility of the material or polymer, and thermal stability.The material should not cause corrosion of the underlying aluminum orother metal circuit if it is used in the production of semiconductordevices. Further, the material should be capable of being uniformly andthinly coated and should be effectively usable in the formation of asingle resist layer as well as an upper layer of the duplitized resistcoating. Use of the resist material in the formation of thethree-layered resist coating is not contemplated, since the coatingcomprises a substrate having coated thereon a leveling layer, a plasmaetching-resistant layer, and a resist layer and therefore itnecessitates lots of complicated and troublesome process steps.

SUMMARY OF THE INVENTION

It was found that a pattern-forming resist material which consists ofpolysilsesquioxane having no hydroxyl group in its molecule has animproved sensitivity to high-energy radiation of up to an order of about1 μC/cm² and a high resolution capability of about 0.3 to 0.5 μm l/s.The resist material is stable at a high temperature. It does not hardenfor about one hour at a temperature of less than 300° C., for example.Since the resist coating is not hardened during prebaking at about 100°C. to 120° C., wherein the solvent is evaporated and adhesion to theunderlying layer to be processed improved, the molecular weight ofpolysilsesquioxane and therefore the sensitivity and resolutioncapability of the resist coating can be maintained withoutdeterioration. The resist material can be effectively used in theproduction of LSI's, very-large-small integrated circuits (VLSI's) andother semiconductor devices.

The polysilsesquioxane of this invention suitable as the resistmaterial, particularly as a negative-working resist material, ispreferably that represented by the formula: ##STR4## in which

R₁ and R₂ may be the same or different and each represents a substitutedor unsubstituted alkyl group such as methyl, chloromethyl, or ethyl, asubstituted or unsubstituted aryl group such as phenyl, chlorophenyl, ortolyl, or a substituted or unsubstituted vinyl group, and m is apositive integer of about 25 to 4,000. The weight-average molecularweight (Mw) of this polymer is about 3,000 to 500,000.

More preferably, the polysilsesquioxane is silylated polysilsesquioxaneof the formula: ##STR5## in which R₁, R₂ and m are as defined above, andR₃ represents a substituted or unsubstituted alkyl group or asubstituted or unsubstituted aryl group. A typical example of thepreferred silylated polysilsesquioxane is trimethyl-silylatedpolysilsesquioxane of the formula: ##STR6## in which R₁, R₂ and m are asdefined above.

According to this invention, there is also provided a process for theproduction of a high-energy radiation-sensitive pattern-forming resistmaterial, which comprises condensing a trifunctional siloxane andremoving an active hydrogen of the remaining unreacted hydroxyl groupsin the molecule, particularly its end portions and/or other portions, ofthe resulting condensate, thereby forming polysilsesquioxane having nohydroxyl group in its molecule.

In the production of the above-described resist material, the removal ofthe unreacted hydroxyl groups from the condensation product can beeffectively carried out by silylating the condensation product. A usefulsilylating agent is monohalogenated silane of the formula: ##STR7##wherein R₄ may be the same or different and each represents hydrogen, asubstituted or unsubstituted alkyl group or a substituted orunsubstituted aryl group, and X is a halogen such as chlorine. Typicalexamples of the preferred silylating agents are (CH₃)₃ SiCl,(ClCH₂)(CH₃)₂ SiCl, (BrCH₂)(CH₃)₂ SiCl, (C₆ H₅)(CH₃)₂ SiCl, [(CH₃)₂ Si]₂O and NC(CH₂)₃ (CH₃)₂ SiCl. The silylating process can be preferablycarried out by using trimethylchlorosilane. The active hydrogen in theremaining hydroxyl group is reacted with the chlorine in the silanecompound to form hydrogen chloride, which is then removed from thereaction system.

According to this invention, there is also provided a process for theformation of a resist pattern on the underlying substrate, generally ofa semiconductor, or layer to be processed, which comprises the steps of:coating on the substrate or layer a solution of the resist materialwhich consists of polysilsesquioxane having no hydroxyl group in itsmolecule; drying the coated resist material in an inert gas; exposingthe dried coating of the resist material to a desired pattern ofhigh-energy radiation for patterning; and developing the patternedcoating of the resist material.

The resist material used in the pattern formation process of thisinvention is preferably polysilsesquioxane represented by the formula(I) above, and more preferably the silylated polysilsesquioxane of theformula (II) or (IIA) above.

In a preferred embodiment of this invention, the pattern formationprocess may further comprise dry etching the underlying substrate orlayer through a mask, namely, a pattern-wise developed coating of theresist material. The dry etching may be carried out by conventionalmeans, such as plasma etching, reactive ion etching, or sputter etching.

According to this invention, there is provided another pattern formationprocess which comprises the steps of: coating on the substrate or layera solution of a lower layer-forming first resist material which consistsof an organic resin; drying the coated first resist material in an inertgas; further coating on the dried coating of the first resist material asolution of the upper layer-forming second resist material whichconsists of polysilsesquioxane having no hydroxyl group in its molecule;drying the overcoated second resist material in an inert gas; exposingthe dried coating of the second resist material to a desired pattern ofhigh-energy radiation for patterning; developing the patterned coatingof the second resist material; dry etching the underlying coating of thefirst resist material through a pattern-wise developed coating of thesecond resist material which acts as a mask, thereby transferring apattern of the second resist coating to the underlying first resistcoating.

In this pattern formation process, the organic resin suitable for theformation of the lower or first resist layer is preferably phenol resin,polyimide resin, or polystyrene resin. Epoxy resin, novolak resin, orrelated organic resins are also useful. Novolak resin or photoresist iscommercially available, for example, from Shipley Co. under thetradename: Microposit 1350. The organic resin, however, is not limitedto those resins, so long as it has no sensitivity to radiation used inthe patterning of the upper resist layer, is not removed duringdevelopment of the exposed upper resist layer, and has a high resistanceto dry etching used to selectively etch the underlying substrate orlayer. Of course, the organic resin should have no resistance to oxygenplasma etching, when such etching is used to transfer a pattern of theupper resist layer to the lower resist layer of the organic resin.

Further, the resist material suitable for the formation of the upper orsecond resist layer is preferably polysilsesquioxane of the aboveformula (I), more preferably, silylated polysilsesquioxane of the aboveformula (II) or (IIA).

In the practice of the pattern formation process using the duplitizedresist coating, the upper resist layer should be thinner while the lowerresist layer should be thicker. The use of a thin upper layer iseffective for obtaining a fine resist pattern having a high sensitivityand resolution capability. In addition to this, as a result of accuratetransfer of the pattern of the upper layer to the dry etching-resistant,thick lower layer, a satisfactorily high resistance to dry etching isalso obtained. Further, since the lower resist layer is thick, it ispossible to level the uneven features appearing on the surface of thesubstrate or layer to be selectively etched. The thickness of the upperresist layer is preferably about 0.2 to 0.5 μm and that of the lowerresist layer preferably about 1.5 to 3.0 μm.

Further, as in the above-described pattern formation process using asingle-coated resist layer, the pattern formation process may furthercomprise dry etching the underlying substrate or layer through apattern-wise etched coating of the first resist material which acts as amask. The coating of the second resist material remaining on the firstresist coating is generally removed during the dry etching step.

Further, according to this invention, there is also provided a processfor the production of semiconductor devices using a lithographytechnique, which comprises dry etching the material to be etched througha mask formed from a high-energy radiation-sensitive pattern-formingresist material consisting of polysilsequioxane having no hydroxyl groupin its molecule, as discussed above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a), 1(b), and 1(c) are cross-sectional views showing, insequence, the prior art pattern formation process using a single resistlayer;

FIGS. 2(a), 2(b), and 2(c) are cross-sectional views showing insequence, the prior art pattern formation process using a duplitizedresist coating,

FIGS. 3(a), 3(b), and 3(c) are cross-sectional views showing, insequence, the pattern formation process of this invention using a singleresist layer;

FIGS. 4(a) through 4(f) are cross-sectional views showing, in sequence,the pattern formation process of this invention using a duplitizedresist coating;

FIG. 5 is a graph of a relation between the molecular weight ofsilylated polymethylsilsesquioxane (PMSS) and its electron beam (EB)sensitivity;

FIG. 6 is a graph of the sensitivity characteristics of PMSS havingdifferent molecular weights;

FIG. 7 is a graph of the range of good solvents for PMSS of thisinvention;

FIG. 8 is a graph of the thickness of the residual PMSS and Microposit1350 resist as a function of the etching time;

FIG. 9 is a graph of the relation between the etch rate and the etchingconditions;

FIG. 10 is a graph of the relation between the depth of the undercut andthe oxygen gas pressure; and

FIG. 11 is a graph of the relation between the thickness of the residuallayer and the EB exposure dose.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As previously described, we found that the objects of this invention areattained by treating any unreacted hydroxyl group remaining in themolecule of the condensation product of trifunctional siloxane tosubstitute its active hydrogen with a suitable substitutent, such as asilyl group.

Generally, polysilsesquioxane can be easily produced by condensingtrifunctional siloxane. For example, polymethylsilsesquioxane can beproduced by the following reaction schema: ##STR8## Namely,methyltrichlorosilane (IV) is first subjected to hydrolysis, and thenthe resulting hydrolyzate (V) is condensed to formpolymethylsilsesquioxane (VI). However, the finally resultingcondensation product is not polymethylsilsesquioxane of the aboveformula (VI), but, in fact, polymethylsilsesquioxane having thefollowing portion, for example: ##STR9## from which it is apparent thatthere are unreacted hydroxyl groups in the molecule of the condensationproduct.

Our study of polymethylsilsesquioxane (VII) revealed the fact thatunreacted hydroxyl groups contained in the molecule of ladder-typesilicone resin or polysilsesquioxane can adversely affect thecharacteristics of polysilsesquioxane in the resist process, therebyresulting in drawbacks discussed in the description of the related priorart. Based on the above, we found that polysilsesquioxane having nohydroxyl group in its molecule, particularly silylatedpolysilsesquioxane, is useful as a pattern-forming resist material.

Further, we studied the production of the described polysilsesquioxaneand its use as the resist for high energy radiation exposure, withreference to PMSS. The results are described in the following examples.

Example 1

This example is included to explain the production of PMSS according tothe reaction schema (1) and (2) described above.

A reactor containing 540 ml of methyl isobutyl ketone (MIBK) as asolvent and 84 ml of triethylamine (TEA) was bubbled and saturated withnitrogen gas. The solution was then cooled, and 78 ml of methyltrichlorosilane (MTCS) was added to the solution. While maintaining thelower temperature of the solution, about 100 ml of ion-exchanged waterwas added dropwise to cause the hydrolysis reaction (1) ofmethyltrichlorosilane. After completion of the addition of water, thesolution was gradually heated and the warm solution was maintained forabout 90 minutes. Thereafter, the solution was further heated to causethe condensation reaction (2). Infrared (IR) spectrophotometric analysisof the resulting condensation product indicated that the product has thestructure of formula (VII) described above. Further, gel permeationchromatography (GPC) using a column of polystyrene gel showed that theproduct (VII) has a molecular weight of about 20,000. The product,however, hardened after heating at about 80° C. for 10 minutes, becauseof the presence of the unreacted hydroxyl groups.

Ten g of the above-prepared product (VII) was dissolved in 100 ml oftoluene to start the silylation. To the solution, 20 ml of pyridine wasadded, and 10 g of trimethyl-chlorosilane (TMCS) was dropped. Thereaction was maintained at a temperature of 60° C. for 2 hours. Afterthe reaction, the reaction product was repeatedly washed each time with50 ml of water to completely remove the pyridine salt. After washingwith the water, acetonitrile was added to the product until no furtherprecipitate was formed. The final product was the desired PMSS having amolecular weight of 20,000 and a degree of dispersion of 1.5. PMSS didnot harden after heating at 200° C. for 1 hour.

In this example, it is believed that the silylation process proceededaccording to the following reaction schema: ##STR10##

EXAMPLE 2

The silylation procedure of Example 1 described above was repeatedexcept that the reaction product (VII) was replaced with low molecularweight (Mw=4,000) polymethylsilsesquioxane having unreacted hydroxylgroups in its end portions. The polymethylsilsesquioxane used in thisexample started to harden after heating at 60° C. for 10 minutes.

The resulting PMSS had a molecular weight of 4,000 and a degree ofdispersion of 1.2. PMSS did not harden after heating at 105° C. for 1hour.

In this example, it is believed that the silylation process proceededaccording to the following reaction schema: ##STR11##

Further, as a result of study of the production of PMSS, it wasdetermined that: (1) Mixing of methyltrichlorosilane (MTCS) with waterat an ordinary temperature results in gelation of the reaction productdue to the rapid reaction. The gelation of the product, however, can beeffectively inhibited if the starting materials are mixed at a lowertemperature and under controlled reaction conditions. (2) Thesensitivity of PMSS is generally increased with an increase of itsmolecular weight. High polymerization of PMSS can be attained if thecondensation is carried out under pressure. (3) Reproducible productionof PMSS can be attained if the fractional precipitation process iscarried out using, for example, isopropyl alcohol (IPA) as a goodsolvent or non-polar solvent and methanol as a bad solvent or polarsolvent. (4) Preparation of monodispersed PMSS is effective to increasethe resolution capability of the pattern-forming material. Suchpreparation is carried out by using a fractional precipitation process,which generally depends on two factors, namely, the hydroxyl equivalentand molecular weight of the polysilsesquioxane to be monodispersed.According to this invention, monodispersed PMSS having a degree ofdispersion of less than 2.0 can be easily prepared, thereby resulting infine resist patterns having a resolution capability of less than 0.5 μml/s. This is because PMSS of this invention has no hydroxyl group andtherefore the fractional precipitation process used in the preparationof monodispersed PMSS can be carried out just based on the differencesof the molecular weight. In contrast, the prior art polysilsesquioxanehaving hydroxyl groups is not suitable for the formation ofmonodispersed PMS.

EXAMPLE 3

This example is included to explain the use of PMSS of this invention asa single electron beam resist, referring to FIGS. 3(a), 3(b), and 3(c).

PMSS (Molecular weight: 20,000; Degree of dispersion: 1.5) produced inExample 1 was dissolved in toluene. The toluene solution of PMSS, as isshown in FIG. 3(a), was spin-coated on a silicon wafer or substrate 1 ata layer thickness of 0.5 μm. The PMSS resist layer 5 was then dried at80° C. for 15 minutes in a gas stream of nitrogen. After drying, thesilicon wafer was conveyed into an electron beam exposure device (notshown) and was exposed to a pattern of electron beams at an accelationvoltage of 20 kV. The irradiation of electron beams is shown with thereference symbol e⁻ in FIG. 3(b). The irradiated area 15 of the resistlayer was cross-linked and therefore insolubilized to a developer usedin the subsequent development step. The silicon wafer was then dipped ina solution of methyl isobutyl ketone (MIBK) for one minute to developthe irradiated area 15 of the resist layer. A fine resist pattern 15having a resolution capability of 0.5 μm l/s was obtained (see FIG.3(c)). The sensitivity was 7.0×10⁻⁶ C/cm².

EXAMPLE 4

The procedure of Example 3 was repeated, except that PMSS of Example 1was replaced with PMSS (Molecular weight: 4,000; Degree of dispersion:1.2) produced in Example 2 described above. A fine resist pattern havinga resolution capability of 0.3 μm l/s was obtained. The sensitivity was4.0×10⁻⁵ C/cm².

EXAMPLE 5

This example is included to explain the experiments which were made toascertain applicability of PMSS, as an electron beam resist, to thesingle-coat resist process and to evaluate the sensitivity andresolution capability of PMSS.

Four samples of PMSS described in the following Table 1 were prepared inthis example. Fractionation of these PMSS samples was made using4-methyl-2-pentanone as a good solvent and acetonitrile as a badsolvent. The distribution of the molecular weight of the fractionatedPMSS was determined by GPC using a column of polystyrene gel and amobile phase of tetrahydrofuran (THF) and at a flow rate of 1.5 ml/min.

                  TABLE 1                                                         ______________________________________                                        Sample   .sup.--Mw     .sup.--Mn                                                                              .sup.--Mw/.sup.--Mn.sup.1                     ______________________________________                                        PMSS-1    1 × 10.sup.4                                                                          9 × 10.sup.3                                                                    1.1                                           PMSS-2    3 × 10.sup.4                                                                         26 × 10.sup.3                                                                    1.2                                           PMSS-3   14 × 10.sup.4                                                                         28 × 10.sup.3                                                                    5.0                                           PMSS-4   40 × 10.sup.4                                                                         93 × 10.sup.3                                                                    4.3                                           ______________________________________                                         .sup.1 .sup.--Mw/.sup.--Mn: Degree of dispersion                         

Each sample of PMSS described above was dissolved in4-methyl-2-pentanone. The PMSS resist solution was spin-coated on asilicon substrate and then prebaked at 80° C. for 20 minutes in a streamof nitrogen gas. The dried resist layer had a thickness of 1.0 μm. Theresulting resist samples were tested to ascertain the relation betweenthe molecular weight of PMSS and its sensitivity or resolutioncapability and the relation between a developing solution or rinsingsolution used and a resolution capability of the resist pattern. Theresults are described hereinafter.

FIG. 5 shows the relation between the weight-average molecular weight(Mw) of PMSS and its sensitivity (D_(g) ^(i)). The graph of this figureindicates that PMSS (Mw=3×10⁴) has an EB sensitivity of 1.6 μC/cm²,while high-molecular weight PMSS (Mw=4×10⁵) has a high EB sensitivity of0.2 μC/cm². This means that the sensitivity of PMSS varies dependingupon its molecular weight.

Further, it is theoretically demonstrated that the relation betweensensitivity and molecular weight satisfies the formula: (Mw)×(G_(g)^(i))=const. Since the product (Mw) and D_(g) ^(i)) is the reactivity ofPMSS to an electron beam, it can be said that the smaller the product of(Mw) and (D_(g) ^(i)), the higher the sensitivity of PMSS. Table 2 shows(Mw)×(D_(g) ^(i)) values of typical negative-working EB resists.

                  TABLE 2                                                         ______________________________________                                        Type of resist    .sup.--Mw × D.sub.g.sup.i (C/cm.sup.2)                ______________________________________                                        Present invention                                                             PMSS-4            0.08                                                        For duplitized resist coating                                                 SNR.sup.1         0.11                                                        P(SiSt--CMS).sup.1                                                                              0.32                                                        For single resist layer                                                       CMS.sup.2         0.13                                                        PGMA.sup.3         0.028                                                      ______________________________________                                         .sup.1 Previously cited                                                       .sup.2 Chloromethylated polystyrene                                           .sup.3 Polyglycidyl methacrylate                                         

Table indicates that PMSS-4 of this invention has a low (M)×(D_(g) ^(i))value. The value is next lowest after that of PGMA, which is a prior artresist having a remarkably high sensitivity and the lowest among threesilicone resists described in the table. The reason why PMSS having nofunctional group shows a remarkably high sensitivity is considered to bethat it contains silicon atoms whose molecular weight is higher thanthat of the carbon atom, and the ratio of the silicon atoms contained inPMSS is higher than that of the other resist materials, and thereforeshows notable internal scattering of electrons in the resist layer.

In addition, FIG. 6 shows a sensitivity curve of each of PMSS-2, PMSS-3,and PMSS-4. From these graphs, it is clear that the higher the molecularweight of PMSS, the higher the sensitivity.

As is apparent from the above discussion, PMSS of this inventionexhibits a molecular weight and sensitivity or resolution capability ofa relation similar to that of the prior art negative-working resist.When the molecular weight of PMSS is increased, the sensitivity isincreased, but the resolution capability is decreased. Namely, thesensitivity and the resolution capability are in an inverse relation.However, when the resolution capability of PMSS is compared with that ofthe prior art resist, its decrease with the increase of the molecularweight is less than that of the prior art, since PMSS is a rigid highpolymer having a ladder structure and no or little interlock of themolecular chains and therefore is rapidly soluble in a solvent.

Based on the above results of experiments, it was attempted to furtherincrease the resolution capability of PMSS-4 (Mw=4×10⁵) through theoptimization of the developing and rinsing solutions used in thesubsequent steps.

FIG. 7 is a graph of the range of good solvents for PMSS-2 and PMSS-4 ofthis invention. The graph indicates that the solubility of PMSS in asolvent varies remarkably depending upon its molecular weight. PMSS, ifits molecular weight is increased, becomes hardly soluble in ketone-typesolvents. In fact, PMSS-4 (Mw=4×10⁵) is insoluble in acetone andcyclohexane. Generally, the polymeric compounds, when their molecularweight is increased, tend to show a decreased solubility due tolimitation of their movement in the solution. PMSS of this invention hasthis tendency, too. The tendency of PMSS is remarkably greater than thatof the polymeric compounds described above. It is believed that thesolubility of PMSS is excessively decreased with an increase of itsmolecular weight, because the limitation on the movement of the PMSSmolecule is larger than that of the prior art straight-chain polymericcompounds due to the rigid, ladder-type main chains of PMSS.

In the formation of resist patterns, the exposed resist coating wasdeveloped with a developing solution of (a) hexane, (b) xylene, or (c)4-methyl-2-pentanone and then rinsed it with isopropyl alcohol (IPA). Incase (a), using hexane as the developer, the resulting resist pattern ofPMSS-4 swelled due to the high solubility of PMSS-4 in hexane and aplurality of resist bridges were formed. For cases (b) and (c), usingxylene and 4-methyl-2-pentanone, respectively, the formation of resistbridges could be inhibited, but much developing residues were produced.Comparing case (b) with case (c), the use of 4-methyl-2-pentanone ismore suitable for the formation of a rectangular resist pattern than theuse of xylene.

From the above results, it is clear that, among the three developingsolutions described above, 4-methyl-2-pentanone is the most suitabledeveloper for the formation of satisfactory resist patterns.

Further, in order to ascertain the relation between a rinsing solutionand a configuration or profile of the resulting resist pattern, thefollowing experiments were conducted

    ______________________________________                                        Experiment No.                                                                            Developing solution                                                                          Rinsing solution                                   ______________________________________                                        1           4-methyl-2-pentanone                                                                         --                                                 2           "              Acetone                                            3           "              Mixture of                                                                    acetone and IPA                                                               (vol. ratio 1:1)                                   4           "              IPA                                                ______________________________________                                    

The results of Experiment No. 1 without rinsing showed that the resistpattern swelled and did not resolve even a space of 3 μm. In contrast,Experiment Nos. 2, 3, and 4 with rinsing showed the increase of theresolution capability of the PMSS resist. As a result of theexperiments, the mixture of acetone and IPA (volume ratio of 1:1) wasfound to be useful to control the rinsing effects and to obtainsatisfactory resist patterns.

EXAMPLE 6

This example is included to explain the use of PMSS of this invention asan upper resist layer of the duplitized resist coating in the formationof a mask of tantalum for X-ray exposure. PMSS of this invention, aspreviously explained, is a high sensitivity resist material. FIGS. 4(a)through 4(f) are referred to.

A 0.8 μm thick tantalum (Ta) coating 6 was deposited on a silicon waferor substrate 1 through a sputtering technique. Then, in order to form alower resist layer of the duplitized resist coating, the photoresistMicroposit 1350, commercially available from Shipley Co., wasspin-coated on the tantalum coating 6 and baked at 200° C. for one hourin a gas stream of nitrogen. The layer thickness of the lowerphotoresist layer 7 was 0.9 μm. After the baking, a solution of PMSS(Mw=25,000) of this invention in 4-methyl-2-pentanone was spin-coated onthe photoresist layer 7 and prebaked at 80° C. for 20 minutes in a gasstream of nitrogen. The resulting upper PMSS layer 8 had a layerthickness of 0.1 μm. The layer structure of the wafer is shown in FIG.4(a).

In the patterning step illustrated in FIG. 4(b), the silicon wafer waspattern-wise irradiated with an electron beam e⁻ at an acceleratingvoltage of 20 KV. Upon the electron beam exposure, the irradiated area18 of the upper PMSS layer was insolubilized.

Thereafter, the silicon wafer was developed by dipping it in methylisobutyl ketone (MIBK) for 30 seconds. The nonirradiated area of theupper PMSS layer was therefore removed. Subsequent to the development,the silicon wafer was rinsed in isopropyl alcohol (IPA) for 30 secondsand then postbaked at 80° C. for 15 minutes in a gas stream of nitrogen.The result is illustrated in FIG. 4(c).

Thereafter, for the pattern transfer purpose, the silicon wafer wasconveyed into a dry etching device of a parallel plate-shaped electrodetype and dry etched with oxygen plasma (Gas pressure: 2 Pa; Appliedpower density: 0.22 W/cm²) for 5 minutes. As illustrated in FIG. 4(d),the pattern of the upper PMSS layer 18 was transferred to the underlyingphotoresist layer 7. The upper PMSS layer 18 acted as an intermediatemask in the dry etching step.

After completion of the oxygen plasma etching, as illustrated in FIG.4(e), the tantalum coating 6 was etched with the plasma of the mixed gas(C Cl₄ plus CF₄ /2:1) using a mask of the lower photoresist layer 7. Thegas pressure was 8 Pa, the applied power density was 0.33 W/cm², and theetching time was 4 minutes. The upper PMSS layer 18 was removed duringthe dry etching step.

Finally, the remaining mask 7 of the photoresist was removed using aconventional removal technique. As is shown in FIG. 4(f), the siliconwafer having the patterned tantalum coating 6 was obtained. A resolutioncapability of the tantalum patterns was 0.25 μm l/S. The sensitivity was10 μC/cm².

The resulting patterns of tantalum on the silicon wafer is unexpectedlyeffective as a mask for X-ray exposure, since the X-rays are notpermeated to the tantalum patterns.

In contrast, hereinbefore, it was difficult to produce an X-ray maskconsisting of tantalum patterns of submicron resolution, if the tantalummask was produced using a electron beam exposure method and a singleresist layer. This is because tantalum (atomic number 73, and atomicweight 181) shows a large electron scattering characteristic or power,and therefore results in unacceptably large backscattering of theelectron beam within the single resist layer. Finally, the tantalum forX-ray exposure could not be produced.

EXAMPLE 7

This is a comparative example.

The procedure of Example 6 described above was repeated, except for thefollowing changes:

(1) The photoresist CMS-EX (chloromethylated polystyrene) commerciallyavailable from Toyo Soda Mfg. Co., Ltd. was used as a resist. It wasspin-coated and prebaked at 80° C. for 20 minutes in a gas stream ofnitrogen to obtain a 1 μm thick resist layer.

(2) The upper resist layer of PMSS was omitted.

(3) The EB exposed resist layer was developed by dipping the siliconwafer in acetone for 60 seconds and then rinsing it in isopropyl alcohol(IPA) for 30 seconds.

The resulting resist patterns had unacceptable bridges extending overthe adjacent patterns. The tantalum patterns had a resolution capabilityof more than 1.5 μm l/s.

EXAMPLE 8

This example is included to explain resist characteristics obtained whenPMSS of this invention having a high glass transition temperature (Tg)is used as an upper layer-forming resist material in the duplitizedresist process.

Novolak photoresist Microposit 1350 commercially available from ShipleyCo. was coated on a silicon wafer with a spin-coater and then baked at200° C. for one hour in a gas stream of nitrogen. A lower resist layerhaving a thickness of 1.5 μm was formed. Next, 0.4 g of PMSS wasdissolved in 1.5 ml of toluene. The resulting upper layer-forming resistsolution was spin-coated on the previously formed lower resist layer andbaked at 80° C. for 10 minutes in a gas stream of nitrogen. The drythickness of the upper resist layer was 0.5 μm.

The silicon wafer with the duplitized resist coating described above waspattern-wise exposed to an electron beam using an acceleration voltageof 20 KV. The exposure dose was controlled by varying the exposure timeat a constant beam current. After the EB exposure, the upper resistlayer was developed by dipping the wafer in a mixed solution of xyleneand o-dichlorobenzene (3:1) at a temperature of 23° C. for 90 to 120seconds. Thereafter, the lower resist layer was dry etched with oxygenplasma using a mask of the patterned upper resist layer. The appliedpower density was 0.33 W/cm², gas pressure was 8 Pa, and etching timewas 10 minutes. The pattern of the upper resist layer was transferredexactly to the underlying lower resist layer.

PMSS and Microposit 1350 were tested as to the thickness of the residuallayer as a function of the etching time. The results are plotted in FIG.8. FIG. 8 indicates that a PMSS resist layer does not decline in layerthickness within the first 10 minutes of oxygen plasma etching, namely,it has a high resistance to oxygen plasma. In contrast, the Microposit1350 resist layer (1.5 μm thickness) was gradually etched with time andwas completely removed in about 6 to 7 minutes.

Further, it was learned that PMSS (Mw=83,000) has a higher sensitivityof 1 μC/cm² and a high resistance to oxygen plasma and that PMSS, if itis used as a upper layer of the duplitized resist coating, results in apattern having a resolution capability of 0.9 μm l/s.

EXAMPLE 9

This example is included to further explain resist characteristicsobtained when PMSS of this invention is used as an upper layer of theduplitized resist coating.

Polyvinyl phenol (Mw=4900) and o-cresol-novolakepoxy resin were mixed.Next, 0.5% by weight of benzimidazole was added as a hardeningaccelerator to a mixture having a functional group equivalent ratio of1:1. The resulting mixture was dissolved in cyclohexane to prepare aleveling solution. The leveling solution was spin-coated on a siliconsubstrate and baked at 200° C. for 30 minutes in a gas stream ofnitrogen. A leveling layer having a layer thickness of 2.0 μm wasformed. Thereafter, a solution of PMSS resist was spin-coated on theleveling layer and baked at 80° C. for 20 minutes in a gas stream ofnitrogen. A 0.2 μm thick upper resist layer was formed.

The silicon wafer with the duplitized resist coating described above waspattern-wise exposed to an electron beam using an acceleration voltageof 20 KV. The EB exposure dose was controlled by varying the exposuretime at a constant beam current. After the EB exposure, the upper resistlayer was developed by dipping the wafer in 4-methyl-2-pentanone for 60seconds and then, in order to remove the developer, rinsing the wafer bydipping it in a mixed solution of acetone and isopropyl alcohol (IPA)(1:1) for 30 seconds.

After patterning of the upper resist layer was completed, the underlyinglower resist or leveling layer was dry etched with oxygen plasma using amask of the patterned upper resist layer. The etching was made totransfer a pattern of the upper resist layer to the leveling layer.

In the dry etching step, the etch rate was determined after the 2 μmthick leveling layer was etched for a predetermined period. Further, thedepth of the undercut or lateral etch was determined using a scanningelectron microscope (SEM), after completion of the dry etching of theleveling layer with oxygen plasma using a mask of the patterned upperresist layer.

In order to ascertain the variation of the etch rate due to etchingconditions, the leveling layer was dry etched using different oxygen gaspressures and applied power densities. The results are plotted in FIG.9. FIG. 9 indicates that the etch rate increases with increase of theapplied power density, and that a peak of the etch rate is attained whenthe oxygen gas pressure is from 10 to 13 Pa. It is believed that a peakof the etch rate curve is formed due to an increase or decrease of theoxygen gas pressure. When the gas pressure is higher, the energybestowed to the individual oxygen radical is smaller, however the totalnumber of the oxygen radicals is smaller.

Further, in order to ascertain the variation of the depth of theundercut or lateral etch depending upon the particular oxygen gaspressure, the leveling layer was dry etched using different oxygen gaspressures. Two power densities of 0.22 and 0.33 W/cm² were applied inthe dry etching process. The results are plotted in FIG. 10. FIG. 10,showing the variation of the depth of the undercut as a function of theoxygen gas pressure, indicates that the lower the oxygen gas pressure orthe higher the applied power density, the smaller the depth of theundercut. For example, when a power density of 0.22 W/cm² is applied,pattern transfer at an oxygen gas pressure of 2.0 Pa results in a smallundercut of 0.07 μm. The depth of the undercut is increased withincrease of the gas pressure. The depth, at a gas pressure of about 10to 13 Pa, at which pressure the etch rate of the leveling layer showsthe highest value, is about 0.25 μm. It is, therefore, considered that,when the depth of the undercut is desired to be reduced to attain a highdimensional accuracy, the etching of the leveling layer must be carriedout at a reduced gas pressure and at the same time the etch rate must belowered. Further, when the applied power density is increased to attaina higher etch rate, much resist residues are formed on the substrate. Wefound that if the etching of the leveling layer is carried out at anoxygen gas pressure of 2.0 Pa and applied power density of 0.22 W/cm²,the transfer time of the upper resist pattern to the leveling layer isshortened to about 16 minutes and the depth of the undercut isdiminished to about 0.07 μm.

The duplitized resist coating comprising an upper PMSS-4 layer (0.2 μmthick) and a lower leveling layer (2.0 μm thick) was prepared, andtested as to its sensitivity characteristics. The results are plotted inFIG. 11. FIG. 11 indicates that PMSS-4, when it is used as an upperlayer of the duplitized resist coating, shows a remarkably improvedsensitivity of 0.8 μC/cm².

The exposed upper PMSS-4 layer was developed with 4-methyl-2-pentanoneand then rinsed with a mixed solution of acetone and isopropyl alcohol(IPA) (1:1). Thereafter, the lower leveling layer was dry etched withplasma oxygen at an oxygen gas pressure of 2.0 Pa and applied powerdensity of 0.22 W/cm². For the silicon substrate, a resist pattern of0.5/0.7 μm l/s was formed on the substrate. Further, for the aluminumsubstrate, a resist pattern of 2.0/0.5 μm was formed. Furthermore, thesesubmicron patterns showed a high aspect ratio of 4.4.

EXAMPLE 10

This example is included to further explain the use of PMSS of thisinvention as an upper layer of the duplitized resist coating for X-rayexposure.

The procedure of Example 6 described above was repeated, but, in thisexample:

(1) PMSS used herein was PMSS-2 and PMSS-4.

(2) In the patterning step, the upper PMSS layer was pattern-wiseirradiated with X-rays at 13 kV and 100 mA.

(3) After the X-ray exposure, the wafer was developed with methylisobutyl ketone (MIBK) for 30 seconds, and then rinsed with isopropylalcohol (IPA) for 30 seconds.

(4) For the pattern transfer purpose, the wafer was dry etched withoxygen plasma using a dry etching device of a parallel plate-shapedelectrode type.

As a result, it was determined that the sensitivity of PMSS-2 to X-raywas 160 mJ/cm² and that of PMSS-4 was 15 mJ/cm².

Further, the above procedure was repeated using a polyimide membrane. A0.8 μm thick tantalum pattern as a mask was formed on the polyimidemembrane. After the pattern transfer to the polyimide membrane usingX-ray contact exposure, PMSS-2 showed the resolution capability of 0.3μm l/S, and PMSS-4 showed the resolution capability 0.7 μm l/S.

We claim:
 1. A process for the formation of a resist pattern on asubstrate, which comprises the steps of:coating on the substrate asolution of a resist material for forming said resist pattern, saidresist material consisting of silylated polysilsesquioxane of theformula: ##STR12## in which R₁ and R₂ are the same or different and eachrepresents a substituted or unsubstituted alkyl group, a substituted orunsubstituted aryl group, or a substituted or unsubstituted vinyl group,and m is a positive interger of 25 to 4,000; drying the coated resistmaterial in an inert gas; exposing the dried coating of the resistmaterial to a desired pattern of high-energy radiation for patterning;and developing the patterned coating of the resist material.
 2. Apattern formation process as in claim 1, which further comprises dryetching the underlying substrate through the developed patterned coatingof the resist material which acts as a mask.
 3. A process for theformation of a resist pattern on a substrate, which comprises the stepsof:coating on the substrate a solution of a resist material of whichsaid resist pattern is to be formed, said resist material consisting ofsilylated polysilsesquioxane of the formula: ##STR13## in which R₁ andR₂ are the same or different and each represents a substituted orunsubstituted alkyl, aryl or vinyl group, R₃ is a substituted orunsubstituted alkyl or aryl group, and m is a positive integer in therange of 25 to 4,000;drying the coated resist material in an inert gas;exposing the dried coating of the resist material to a desired patternof high-energy radiation for patterning; and developing the patternedcoating of the resist material.
 4. A pattern formation process as inclaim 3, which further comprises dry etching the underlying substratethrough the developed patterned coating of the resist material whichacts as a mask.
 5. A process for the formation of a resist pattern on asubstrate, which comprises the steps of:coating on the substrate asolution of a lower layer-forming first resist material which consistsof an organic resin; drying the coated first resist material in an inertgas; further coating on the dried coating of the first resist material asolution of an upper layer-forming second resist material consisting ofsilylated polysilsesquioxane of the formula: ##STR14## in which R₁ andR₂ are the same or different and each represents a substituted orunsubstituted alkyl group, a substituted or unsubstituted aryl group, ora substituted or unsubstituted vinyl group, and m is a positive integerof 25 to 4,000,drying the overcoated second resist material in an inertgas; exposing the dried coating of the second resist material to adesired pattern of high-energy radiation for patterning; developing thepatterned coating of the second resist material; and dry etching theunderlying coating of the first resist material through the developedpatterned coating of the second resist material which acts as a mask,thereby transferring a pattern of the second resist coating to theunderlying first resist coating.
 6. A pattern formation process as inclaim 5, which further comprises dry etching the underlying substratethrough the etched coating of the first resist material which acts as amask.
 7. A pattern formation process as in claim 5, in which the organicresin of the lower layer-forming first resist material is phenol resin,polyimide resin, or polystyrene resin.
 8. A process for the formation ofa resist pattern on a substrate, which comprises the steps of:coating onthe substrate a solution of a lower layer-forming first resist materialwhich consists of an organic resin; drying the coated first resistmaterial in an inert gas; further coating on the dried coating of thefirst resist material a solution of an upper layer-forming second resistmaterial which consists of silylated polysilsesquioxane of the formula:##STR15## in which R₁ and R₂ are the same or different and eachrepresents a substituted or unsubstituted alkyl, aryl or vinyl group, R₃is a substituted or unsubstituted alkyl or aryl group, and m is apositive integer in the range of about 25 to 4,000;drying the overcoatedsecond resist material in an inert gas; exposing the dried coating ofthe second resist material to a desired pattern of high-energy radiationfor patterning; developing the patterned coating of the second resistmaterial; and dry etching the underlying coating of the first resistmaterial through the developed patterned coating of the second resistmaterial which acts as a mask, thereby transferring a pattern of thesecond resist coating to the underlying first resist coating.
 9. Apattern formation process as in claim 8, which further comprises dryetching the underlying substrate through the etched coating of the firstresist material which acts as a mask.
 10. A pattern formation process asin claim 8, in which the organic resin of the lower layer-forming firstresist material is phenol resin, polyimide resin, or polystyrene resin.